Suk Saran in Wimbledon (Wimbledon Hill, to be precise) is lovely. We go fairly often and love the gentleness of the waitresses/waiters, the (proper) Thai flavours and the ambience.
I went to the Dairy a while ago for a girlie lunch and, although the food was good, you have to order lots of little dishes to make up a meal - it worked out to be quite an expensive way of doing things. It's worth reading the reviews on Tripadvisor to see if they have solved some of their problems before you make the booking.
Whereever you end up, have a wonderful evening!